Effect of charge transfer state on the exchange interaction of radical-triplet encounter pairs

Abstract

Chemically induced dynamic electron polarization (CIDEP) of galvinoxyl was measured in various excited molecule-galvinoxyl systems prepared by laser photolysis. Most of the systems examined showed net emission CIDEP, which is well explained by the quartet precursor radical-triplet pair mechanism with exchange interaction, J, of negative sign (quartet is higher than doublet). Several systems with molecules such as naphthalene, quinoxaline, biphenyl and triphenylene, however, showed net absorption CIDEP. Time profiles of CIDEP and kinetic analysis of quenching suggest that net absorption CIDEP is generated during the triplet quenching process by the galvinoxyl radical. We conclude that the net absorption CIDEP is produced during the triplet quenching if the J value of radical-triplet encounter pair is positive. This is the first report of the radical-triplet encounter pairs with positive J value. The mechanism for this unusual positive sign of J value is discussed on the basis of the spinselective configuration interaction between the doublet spin correlated states of radical-triplet and charge transfer encounter pairs.

Dynamic electron polarization created by the radical-triplet pair mechanism: Application to the studies on excited state deactivation processes by free radicals

“…The discussions mentioned above are based on RT pairs with the negative J value. If the pair shows the positive J value, the signal sign becomes opposite to that in the pair with the negative J value [16,17]. This relation is summarized in Table 1.…”

“…The former is for the CIDEP created in any radical-triplet (RT) pairs, while the latter is for the CIDEP in radical and spin-polarized triplet systems. The pattern of CIDEP by RTPM in excited state quenching is certainly related to the electron spin dynamics of the quenching process, and efforts to analyze the CIDEP pattem have been made to clarify many details of the potential surfaces of an RT pair and the spin dynamics [9][10][11][12][13][14][15][16][17]. One of the most widely studied subjects on the basis of CIDEP analyses in excited state quenching is the enhanced intersystem crossing (ISC) of both S 1 and T~ molecules promoted by the effects of free radicals.…”

“…Dominant factors to determine a CIDEP pattern are described, in particular, the spin multiplicity of the excited-state molecule [4,5] and the energy separation J of the quartet (IQ)) and the doublet (ID)) spin states of RT encounter pairs [5,16,17] are discussed in detail. After describing the RTPM framework, studies on RT intermolecular interactions carried out by the CIDEP analysis are introduced.…”

Interaction between TOAC free radical and photoexcited triplet chromophores linked to peptide templates

“…A negative polarization (A) is observed in the few cases where the radical and the triplet are rigidly linked in a proper position ensuring zero overlap, 20,21 or when in the RTP there is a consistent contribution of charge transfer from the radical to the triplet partner. 22 As for the multiplet effect, emission at low field and absorp-…”

Developments of magnetic-resonance-related spin chemistry in Japan

“…Usually J is negative, but recent investigations showed that J is positive in the pairs of some triplet molecules with galvinoxy radical [63,64], where an intermolecular CT state interacts with the RTP state only among the doublet spin states. When the blue shift caused by this effect is larger than the energy separation of the doublet and quartet states by the exchange interaction, the sign of the 3"0 value turns over to be positive.…”